CPH | Calibrating Lithology / Mineralogy

Petrophysical Training

Licenses

Calibrating Lithology to Core and Sample Data
It’s time to look at the sample descriptions, strip logs, core descriptions, X-ray diffraction, thin sections, and SEM photographs again. These data sets are the major sources of lithology/mineralogy information. Use this data to determine which minerals to use in your mineral models. Where quantitative data is available, compare quantitative results.

The biggest problem with deterministic lithology models is choosing the appropriate minerals to put into the model. A qualitative check on lithology calculations is to run several models with the same mineral selection. If several models give similar results, you probably have a good mineral mix. If they don't agree, it means the model, the parameters used, or the mineral mix are wrong.

Comparison of Lithology Methods. Note that the 2-mineral model will give silly results in a 3-mineral environment, as shown here in the anhydrite layer, unless appropriate parameters and zoning are applied.

One measure of a good log analysis is that results should match ground truth reasonably well. In the case of mineral volume calculations, ground truth is usually qualitative instead of quantitative. However, quantitative sample descriptions can be made by qualified geologists.

There are some quantitative checks that can be made when core data is available. Matrix density and effective porosity should match equivalent core analysis properties. Check depth control and average the core data over an interval similar to the logging tool resolution.

Comparison of log analysis matrix density (red curve in Track 3) with core grain density (black dots).. Calculated matrix density is slightly lower than core due to gas effect in the upper half of the reservoir.

Another demonstration of an accurate porosity - lithology model is to reconstruct the density, neutron, and PE logs from the log analysis answers using the log response equation. If there is a good match between the reconstructed logs and the original logs (except where bad hole conditions have been compensated for) you can be assured that the mineral analysis is at least reasonable, if not perfect. This only true of course when the minerals input to the model are actually present in the rock sequence. Because the mineral calculation is generally underdetermined, there are a large number of mineral mixtures that could satisfy the log data reconstruction.

Sample descriptions a
Sample descriptions are available on many wells. These will contain a written description of the rock chips extracted from the drilling mud. The description will include dominant mineralogy, accessory minerals, cementing minerals, grain size or texture, pore geometry, porosity estimate, and hydrocarbon shows. Shale or clay, if present, will be mentioned, sometimes with a volumetric estimate in percent. This work is done by observation through a microscope. Samples can be re-logged quantitatively after the initial review.

Samples are well mixed by the mud circulation so these descriptions include rock chips from a fairly large interval. In addition, cavings from above the sampled interval will continue to contaminate deeper samples. Samples also take a long time to reach the surface, so their source depth is not perfectly established. The time taken to reach the surface is called the lag time. Lag time is calculated by comparing estimated borehole volume with mud pump capacity and speed. It is checked periodically by adding a chemical tracer to the mud and measuring how long it takes to detect the tracer back at the surface.

A good wellsite geologist will correlate his description to the shape of the drilling time log. Later, the sample depths may be adjusted to the open hole logs, especially gamma ray, resistivity and density logs. The geologist will also eliminate most caving from the descriptions.

Log analysis lithology plot (left) in a complex sequence, and sample description plot (right) over the same interval. Although the lithology description is not usually quantitative, it is an essential ingredient in choosing the correct mineral mixture for the log analysis lithology calculation. A little care is needed to read these logs. In this case, the word "SAND" describes the rock texture, not its mineralogy. This is a radioactive sand so it must contain feldspar (decomposed granite) and possibly some quartz, as well as the dolomite and anhydrite layers above the sand. Shale, of course must be handled by an appropriate method. In this case, shale cannot be found using the GR inside the radioactive sand interval.

Your log analysis results should show the same dominant minerals where the samples indicate clean sandstone, limestone, dolomite, anhydrite, salt, or mixtures of these minerals. Some shale should show on your analysis where the samples contain shale or clay minerals. A precise match is probably impossible due to the inherent limitations of sample descriptions. At least the samples will eliminate calculation of shale when in fact the zone is a radioactive sandstone or dolomite.

Remember that sample chips are tiny compared to log response volumes and any individual sample may not be representative of the whole reservoir. Cavings, depth control problems, thin beds, and many other unknown factors affect this comparison, so be realistic and use common sense.

Core descriptions
Core descriptions have a better chance of being on depth with the logs and can contain more detail than sample descriptions, especially in thinly bedded formations. The core may demonstrate more detail than the log resolution can follow, so you should try matching to average data over a 2 or 3 foot interval.

11367208W6
S#	Top	Base	Len	Kmax	K90	Kvert	Porosi	GrDen	BkDen	Soil	Swtr	Lithology
	meters	meters	meter	mD	mD	mD	frac	kg/m3	kg/m3	frac	frac
1	2054.35	2054.54	0.19	0.32	0.30	0.08	0.045	2881	2796	0.138	0.138	DOL INTRANHYARGL
2	2054.54	2054.74	0.20	2.45	2.38	0.41	0.100	2737	2563	0.152	0.237	SS F DOL
3	2054.74	2054.91	0.17	20.40	20.00	0.34	0.116	2689	2493	0.147	0.118	SS F CALC
4	2054.91	2055.07	0.16	16.40	16.40	0.83	0.103	2706	2530	0.136	0.136	SS F CALC
5	2055.07	2055.26	0.19	64.50	57.70	40.30	0.145	2683	2439	0.117	0.183	SS F CALC
6	2055.26	2055.48	0.22	60.30	58.80	37.30	0.148	2679	2431	0.124	0.198	SS F
7	2055.48	2055.58	0.10	84.20	80.00	0.01	0.145	2700	2454	0.116	0.206	SS F
8	2055.58	2055.74	0.16	1.77	0.31	0.03	0.037	2736	2672	0.104	0.363	SS F DOL
9	2055.74	2055.89	0.15	10.00	10.00	4.86	0.124	2694	2484	0.156	0.208	SS F DOL
10	2055.89	2056.02	0.13	15.00	14.20	0.36	0.119	2695	2493	0.145	0.232	SS F DOL
11	2056.02	2056.21	0.19	25.40	19.10	0.07	0.099	2721	2551	0.000	0.142	SS F CALC
12	2056.21	2056.30	0.09	15.00	0.01	0.01	0.107	2700	2518	0.188	0.263	SS F
13	2056.30	2056.47	0.17	99.80	98.60	54.70	0.147	2696	2447	0.107	0.246	SS F
14	2056.47	2056.75	0.28	230.00	225.00	164.00	0.158	2679	2414	0.101	0.251	SS F
15	2056.75	2056.93	0.18	189.00	170.00	67.00	0.168	2691	2407	0.098	0.245	SS F CALC
16	2056.93	2057.13	0.20	206.00	198.00	175.00	0.171	2678	2391	0.088	0.296	SS F
17	2057.13	2057.37	0.24	108.00	104.00	94.10	0.166	2658	2383	0.120	0.361	SS F
18	2057.37	2057.55	0.18	152.00	141.20	82.00	0.196	2663	2337	0.115	0.298	SS F
19	2057.55	2057.73	0.18	135.00	135.00	80.00	0.191	2672	2353	0.162	0.246	SS F V/F
20	2057.73	2058.01	0.28	186.00	186.00	80.00	0.207	2659	2316	0.099	0.262	SS F V/F
21	2058.01	2058.26	0.25	37.10	36.50	0.55	0.129	2701	2482	0.095	0.219	SS F DOL
22	2058.26	2058.44	0.18	207.00	181.00	28.60	0.197	2683	2351	0.086	0.224	SS F
23	2058.44	2058.62	0.18	0.90	0.29	0.01	0.022	2737	2699	0.276	0.331	SS F CALC
24	2058.62	2058.77	0.15	271.00	237.00	5.88	0.150	2678	2426	0.081	0.359	SS F CALC
25	2058.77	2058.99	0.22	7.45	7.33	0.12	0.091	2701	2546	0.109	0.146	SS F DOL
26	2058.99	2059.20	0.21	15.70	14.00	0.06	0.098	2700	2533	0.163	0.163	SS F DOL
27	2059.20	2059.42	0.22	27.80	18.89	4.35	0.139	2697	2461	0.162	0.223	SS F DOL
28	2059.42	2059.59	0.17	12.80	12.80	0.05	0.104	2710	2532	0.183	0.160	SS F DOL
29	2059.59	2059.76	0.17	30.90	29.60	0.01	0.075	2720	2591	0.145	0.181	SS F DOL
30	2059.76	2059.88	0.12	77.90	77.10	68.10	0.145	2647	2408	0.086	0.205	SS F
31	2059.88	2060.14	0.26	76.20	72.90	25.50	0.160	2666	2399	0.096	0.221	SS F
32	2060.14	2060.34	0.20	21.50	20.30	0.10	0.185	2777	2448	0.132	0.205	SS F DOL
33	2060.34	2060.47	0.13	12.60	11.80	0.38	0.102	2719	2544	0.177	0.155	SS F DOL
34	2060.47	2060.77	0.30	0.08	0.08	0.01	0.047	2716	2635	0.138	0.255	SS F DOL ARGL
35	2060.77	2060.95	0.18	0.13	0.07	0.01	0.055	2712	2618	0.000	0.535	SS F DOL
36	2060.95	2061.10	0.15		0.01	0.01						SHALE
37	2061.10	2061.26	0.16	0.02	0.01	0.01	0.031	2752	2698	0.000	0.504	SS F DOL ARGL
38	2061.26	2061.52	0.26	0.03	0.01	0.01	0.047	2743	2661	0.000	0.505	SS F DOL
39	2061.52	2061.71	0.19	0.05	0.01	0.01	0.012	2720	2699	0.000	0.622	SS F DOL
40	2061.71	2061.94	0.23		0.01	0.01						SHALE
41	2061.94	2062.07	0.13	0.02	0.01	0.01	0.027	2773	2725	0.000	0.534	SS F DOL
42	2062.07	2062.31	0.24	3.31	3.20	1.32	0.084	2707	2564	0.155	0.217	SS F DOL
43	2062.31	2062.54	0.23	8.80	8.38	3.62	0.121	2735	2525	0.096	0.119	SS F DOL
44	2062.54	2062.67	0.13	12.90	11.30	6.41	0.132	2800	2562	0.103	0.144	SS F DOL ANHY
45	2062.67	2062.92	0.25	0.90	0.80	0.01	0.077	2737	2603	0.108	0.108	SS F DOL
46	2062.92	2063.09	0.17	0.37	0.37	0.04	0.057	2726	2628	0.167	0.232	SS F DOL
47	2063.09	2063.26	0.17	0.54	0.53	0.13	0.080	2731	2593	0.107	0.215	SS F DOL
48	2063.26	2063.39	0.13	1.51	1.40	0.62	0.077	2721	2588	0.134	0.205	SS F DOL
49	2063.39	2063.64	0.25	0.42	0.37	0.17	0.074	2717	2590	0.116	0.143	SS F DOL
50	2063.64	2063.89	0.25	0.29	0.28	0.03	0.074	2741	2612	0.149	0.275	SS F DOL
51	2063.89	2064.00	0.11	0.18	0.16	0.02	0.050	2701	2616	0.084	0.189	SS F DOL
52	2064.00	2064.17	0.17	0.26	0.23	0.09	0.061	2720	2615	0.048	0.240	SS F DOL
53	2064.17	2064.31	0.14	0.16	0.04	0.01	0.031	2772	2717	0.123	0.197	SS F DOL ARGL
54	2064.31	2064.46	0.15	0.13	0.09	0.03	0.059	2743	2640	0.138	0.277	SS F DOL
55	2064.46	2064.64	0.18	0.02	0.02	0.01	0.039	2755	2687	0.000	0.314	SS F DOL ARGL
56	2064.64	2064.80	0.16	0.06	0.05	0.01	0.028	2736	2687	0.000	0.406	SS F DOL
57	2064.80	2064.96	0.16	0.12	0.09	0.01	0.046	2740	2660	0.000	0.270	SS F DOL
58	2064.96	2065.19	0.23		0.01	0.01						SHALE
59	2065.19	2065.39	0.20	0.08	0.05	0.01	0.061	2782	2673	0.000	0.622	DOL P/S ANHYSD
60	2065.39	2065.53	0.14	1.27	1.18	0.10	0.081	2783	2639	0.089	0.221	DOL P/S ANHYSD
61	2065.53	2065.71	0.18	1.78	1.56	0.29	0.074	2802	2669	0.087	0.134	DOL P/S ANHYSD
62	2065.71	2065.87	0.16	0.32	0.32	0.02	0.060	2783	2676	0.000	0.565	DOL P/S ARGLSD
63	2065.87	2066.08	0.21	0.28	0.24	0.04	0.065	2791	2675	0.054	0.108	DOL P/S ANHYSD
64	2066.08	2066.15	0.07	0.18	0.01	0.01	0.077	2780	2643	0.000	0.054	DOL P/S SD CHT
65	2066.15	2066.54	0.39	0.18	0.15	0.01	0.041	2849	2773	0.100	0.125	DOL P/S ANHYSD
66	2066.54	2066.67	0.13	30.90	28.10	2.28	0.156	2815	2532	0.119	0.089	DOL P/S ANHYSD
67	2066.67	2066.82	0.15	13.30	10.20	2.18	0.108	2774	2582	0.115	0.064	DOL P/S SD
68	2066.82	2067.10	0.28	4.62	2.34	1.52	0.092	2787	2623	0.108	0.108	DOL P/S ANHYSD
69	2067.10	2067.34	0.24	0.36	0.19	0.01	0.039	2831	2760	0.047	0.047	DOL P/S H/F ANHYSD
70	2067.34	2067.57	0.23	0.06	0.04	0.01	0.033	2804	2744	0.096	0.479	DOL PP/VANHYSD
71	2067.57	2067.59	0.02		0.01	0.01						SHALE
72	2067.59	2067.86	0.27	0.07	0.07	0.01	0.040	2758	2688	0.139	0.312	SS VF DOL ARGL
73	2067.86	2067.87	0.01		0.01	0.01						SHALE
74	2067.87	2068.04	0.17	0.05	0.05	0.01	0.033	2752	2694	0.092	0.459	SS VF DOL ARGL
75	2068.04	2068.29	0.25	0.16	0.15	0.01	0.067	2761	2643	0.000	0.610	SS F DOL
76	2068.29	2068.54	0.25	0.04	0.04	0.01	0.025	2782	2737	0.000	0.396	SS F DOL
77	2068.54	2068.76	0.22	0.05	0.04	0.01	0.031	2754	2700	0.000	0.284	SS F DOL
78	2068.76	2068.91	0.15	0.02	0.02	0.01	0.027	2795	2747	0.000	0.360	SS F DOL
79	2068.91	2069.10	0.19	0.11	0.03	0.01	0.016	2753	2725	0.000	0.427	SS F DOL
80	2069.10	2069.39	0.29	0.04	0.02	0.01	0.024	2803	2760	0.000	0.600	SS F DOL ARGLPRY
81	2069.39	2069.68	0.29	0.02	0.02	0.01	0.019	2794	2760	0.000	0.382	SS F DOL ANHY

Arithmetic Averages			0.19	33.0	28.8	12.8	0.089	2735	2582	0.091	0.268

Conventional Core Analysis listings are used to determine the choice of minerals for a log analysis lithology calculation. This reservoir is described as a dolomitic sandstone. overlain by an anhydrite cap. The description column on the right shows the variation in the mineralogy versus depth. The Grain density column is used to compare the core data to the log analysis matrix density calculation. The 3-mineral model would include quartz, dolomite and anhydrite. A two mineral model would need to be zoned with dolomite and anhydrite over the cap rock and quartz with dolomite over the sand interval.

Here is the lithology and porosity analysis and core porosity for the same interval as the core listing above.

THIN SECTION PETROGRAPHY
Photo-micrographs of sample chips or portions cut from cores can be interpreted by a petrologist. Results are usually written mineral descriptions with considerable detail. Some can be quantitative. Thin section photographs are made by first injecting a coloured resin into the pores, then slicing and polishing. By passing light through the thin section, particular minerals can be identified by their colour and crystal structure. These can be tabulated numerically and are called thin section point counts.

Clays and shales are easily identified as to quantity and type. One petrological term can be confusing to log analysts. The word “matrix” is used to describe fine-grained minerals (often clays) surrounded and between larger mineral grains. Log analysts use the term “matrix” to mean all the minerals that make up the rock, excluding shale and pore space.


15X Magnification	100X Magnification

Thin Section Images

Depth, ft.	9403.70	9407.00	9413.50	9419.20
Porosity @ NOB (%)	12.4	8.2	10.9	5.0
Air Perm. @ NOB (md)	0.296	0.034	0.338	0.0054
Grain Density (g/cc)	2.81	2.83	2.82	2.79
PRIMARY MINERAL
Dolomite	60.0	81.2	80.0	79.6
Calcite	Tr	0.0	0.0	0.0
Anhydrite	1.2	0.4	0.8	0.0
Pyrite	2.0	1.6	1.6	1.6
Quartz	0.0	0.0	0.0	0.0
Feldspar	0.0	0.0	0.0	0.0
Authigenic Clay	0.0	0.0	0.0	0.0
Bitumen	0.0	0.0	0.0	0.0
Other	0.0	0.0	0.0	0.0
Total	63.2	83.2	82.4	81.2
SILICLASTICS
Mono Quartz	8.8	2.0	4.4	7.2
Poly Quartz	0.0	0.0	Tr	0.0
Plagioclase	2.0	0.8	0.8	1.6
Potassium Feldspar	3.6	1.2	0.8	3.2
Chert	0.0	0.0	0.0	0.0
Rock Fragments	0.0	0.4	0.0	0.0
Shale Fragments	0.0	Tr	0.0	0.0
Muscovite	Tr	0.4	0.0	Tr
Biotite	2.0	0.8	0.0	0.0
Heavy Minerals	0.0	Tr	0.0	0.4
Carbonaceous Fragments	1.2	0.4	Tr	Tr
Glauconite	0.0	0.0	0.0	0.0
Detrital Clay Matrix	3.2	1.6	1.6	1.2
Other	0.0	0.0	0.0	0.0
Total	20.8	7.6	7.6	13.6
POROSITY
Primary Interparticle	0.0	0.0	0.0	0.0
Primary Intraparticle	0.0	0.0	0.0	0.0
Secondary Intraparticle (Carbonate Grains)	0.0	0.0	1.2	0.0
Tertiary Intraparticle (Carbonate Grains)	0.0	0.0	0.0	0.0
Secondary Intraparticle (Siliciclastic)	Tr	0.0	Tr	0.4
Vugular	0.0	0.0	Tr	0.0
Intercrystalline	16.0	9.2	8.4	3.6
Micropores	0.0	0.0	0.0	0.0
Fracture	0.0	0.0	0.4	0.8
Secondary Intracrystalline	Tr	Tr	0.0	0.4
Total	16.0	9.2	10.0	5.2

	100.0	100.0	100.0	100.0

Typical Thin Section Point Count Analysis

X-ray diffraction data (XRD)
X-ray diffraction data (XRD) lists minerals quantitatively. Samples can be very small, so some care must be taken in up-scaling to log resolution. Data tables will list many minerals and various minerals may need to be grouped.

Sample	CLAYS				CARBONATES			OTHER MINERALS						TOTALS
Depth	Chlor-ite	Kaol-inite	Illite	Mixd	Calc-ite	Dolo	Side-rite	Qrtz	K-spar	Plag.	Pyrite	Anhy-drite	Barite	Clays	Carb.	Other
9403.70'	0	0	3	0	Tr	65	0	19	7	6	0	0	0	3	65	32
9407.90'	0	0	2	0	1	91	0	2	2	2	Tr	0	0	2	92	6
9413.50'	0	0	2	0	1	88	Tr	4	2	2	Tr	1	0	2	89	9
9419.20'	0	0	1	0	1	89	0	4	1	2	Tr	2	0	1	90	9
9423.30'	0	0	1	0	0	91	Tr	5	1	1	0	1	0	1	91	8
9425.75'	0	0	2	0	1	90	Tr	3	1	1	Tr	2	0	2	91	7
10354.65'					1	87	0	3	1	1	Tr	7	0	Tr	88	12
10359.95'	0	0	Tr	0	1	93	0	0	Tr	2	Tr	4	0	Tr	94	6
10361.22'					1	83	Tr	Tr	1	1	1	13	0	Tr	84	16
10364.45'					1	86	Tr	0	1	1	Tr	11	0	Tr	87	13
10371.25'					Tr	86	0	4	1	2	Tr	7	0	Tr	86	14
10375.55'					1	94	0	Tr	2	1	Tr	2	0	Tr	95	5
10382.55'					1	79	Tr	6	3	4	1	6	0	Tr	80	20
10384.25'	0	0	Tr	0	Tr	95	0	Tr	1	1	Tr	3	0	Tr	95	5
10390.30'					1	93	0	Tr	Tr	1	0	5	0	Tr	94	6
10395.85'					Tr	62	Tr	1	1	2	0	34	0	Tr	62	38
10399.50'					Tr		Tr	Tr	1	1	Tr	1	0	Tr	97	3

AVERAGE	0	0	1	0	1	86	Tr	3	2	2	Tr	6	0	1	87	12
* Randomly-interstratified mixed-layer illite/smectite

Typical XRD Analysis Results in a Shaly Carbonate

Scanning Electron Microscope (SEM)
Scanning Electron Microscope (SEM) photographs are also used to determine mineralogy. These are more detailed than photo-micrographs and up-scaling is a problem. Written descriptions and numerical listings need to be averaged to match log resolution.

SEM Image 2000 X Magnification

All methods can suffer from non-representative samples (caving or lag time), so ground truth may not be as “true” as we would like. Up-scaling is always a problem. In addition, quantitative data is hard to find even when the work has been done competently.

The final test for mineralogy from log analysis is usually to compare shale corrected porosity with core porosity. If porosity doesn't match and shale volume is considered to be reasonable, then mineral properties or mineral choices may have to be adjusted. This is especially true where only one porosity log is available, since porosity is strongly related to these choices.

THIS PAGE

Calibration Basics
Sample Descriptions
Core Descriptions
Thin Section Petrography
X-Ray Diffraction Data
SEM Photography

THIS CHAPTER

Lithology Basics
Visual Analysis
Selecting Lithology Methods
PE-Density-Neutron Models
Density Log Models
Sonic Log Models
Sonic-Density-Neutron Models
Gamma Ray Spectral Models
Non-Porous Mineral Triggers
Vp/Vs Ratio Method
Simultaneous Equations
Cramer's Rule
Induced GR Spectroscopy Model
Probabilistic Models - Basics
Probabilistic Models - Reality
Principal Components Models
Coding Lithology Plots
Calibrating Lithology
Mineral Properties List

Page Views ---- Since 01 Jan 2015
Copyright 2023 by Accessible Petrophysics Ltd.
CPH Logo, "CPH", "CPH Gold Member", "CPH Platinum Member", "Crain's Rules", "Meta/Log", "Computer-Ready-Math", "Petro/Fusion Scripts" are Trademarks of the Author

Welcome To Crain's Petrophysical Handbook Handbook Index Petrophysical Training About the Author Terms of Use Contact Us Search
CPH Members Have Unrestricted Access. Guests Have Restricted Access. BECOME A CPH GOLD or PLATINUM MEMBER
Site Navigation ☰	MINERALOGY CALIBRATION TO GROUND TRUTH	Quick Links ☰